Computer systems today are subject to a variety of attacks that can disrupt or disable normal operation of a computer system. Computer viruses, worms, and trojan horse programs are examples of different forms of attack. Attacks can also come directly from unscrupulous users of a computer system. Often these attacks take the form of attempts to modify existing program code executed by the computer system or attempts to inject new unauthorized program code at various stages of normal program execution within the computer system. Systems and methods for preventing such malicious attacks are becoming increasingly important.
A typical computer system comprises computer hardware, an operating system, and one or more application programs. The computer hardware typically comprises a processor (sometimes also referred to as a “central processing unit” or “CPU”), a memory, and one or more system buses that facilitate communication among the various components. Other components of a typical computer system include input/output controllers, a memory controller, a graphics processing unit, an audio controller, and a power supply.
Such systems generally have a small amount of on-chip memory (referred to as cache memory) and a much larger amount of off-chip memory (referred to as system memory). The off-chip memory in such systems is generally not considered to be trustworthy (cache memory may also not be considered trustworthy but can be much easier to protect through hardware mechanisms that prevent an attacker from reading the contents of cache memory). That is, data stored in the large system memory is vulnerable to attack wherein the data could be easily altered in a way that was not intended by the owners of the data. Such an attack would cause a program to operate in an unintended manner or allow copy protection scheme to be defeated.
A number of systems have been developed that try to ensure that the data retrieved from system memory can be secured. In particular, systems have employed extensive encryption techniques as well as other tamper evident mechanisms that detect alterations to data in memory.
The operating system can be thought of as an interface between the application programs and the underlying hardware of the computer system. The operating system typically comprises various software routines that execute on the computer system processor and that manage the physical components of the computer system and their use by various application programs.
The processor of a computer system often includes a memory management unit that manages the use of memory by the operating system and any application programs. Many of the attacks against computer systems target programs in memory. For example, portions of code that execute security checks could be defeated by simply replacing that portion of a program when it is stored in memory. Other attacks could modify computer games and change the behavior. For example, consider a situation in which a vulnerability is discovered in a multiplayer game that allows a player to gain an unfair advantage by changing the code on his local machine. Such an unfair advantage could undermine the popularity of an otherwise popular game. All of these considerations suggest that it is highly desirable to prevent unauthorized alterations to program code.